The present invention relates to a method and a device for detecting mercury in a sample solution containing the mercury in cationic form and as an organic compound, by means of a detection device comprising an atomic spectroscope provided with a measuring cell which is suitable for detecting mercury gas, a noble-metal concentrator for concentrating the mercury and a control unit, the mercury being converted into the gas phase and being adapted to be concentrated by means of the noble-metal concentrator.
Mercury is one of the elements having a toxic effect on human beings and animals in a very low concentration. It is therefore necessary to detect mercury concentrations even in very small doses. In addition to the total mercury concentration, especially also the chemical species in which the mercury appears in the sample to be tested are of importance. In this respect, methyl mercury is regarded as one of the most frequently appearing species, having, in addition, an extremely toxic effect.
In conventional methods and devices mercury is detected e.g. by means of the optical atomic spectrometry in the form of atomic absorption spectrometry, atomic fluorescence spectrometry, atomic emission spectrometry or mass spectrometry. In the case of these methods the mercury contained in cationic form or as an organomercury compound in a solution to be tested is determined e.g. by the so-called cold-vapour technique. For this purpose, the mercury contained in the solution is converted by reaction into an atomic gas or vapour, whereupon it is possible to carry out mercury detection by one of the above-mentioned methods in the field of measurement technology.
In order to increase the power of detection, other components in the solution can be eliminated. The sensitivity in the detection process can be increased still further by conducting the mercury gas produced first over a net of noble-metal alloys and by concentrating it then by amalgamation. Subsequently, the mercury adhering to the net is released in the form of mercury gas by heating the net, whereupon it is subjected to the measurement.
By means of these methods it is possible to determine the total mercury content; for a separate determination of e.g. methyl mercury, a previous separation into species will, however, be necessary. This separation normally takes place in a chromatographic method or via cooled condensation traps. This, however, necessitates a separation unit and a detection unit as well as essential additional efforts for preparing the sample so as to carry out this separate determination of the methyl mercury content and of the total mercury content.
Furthermore, a method is imaginable which utilizes the fact that cationic mercury can be converted into the vapour or gas phase with mild reducing agents, whereas certain organomercury compounds can only be converted into the vapour or gas phase with comparatively strong reducing agents. It is true that this method offers an advantage in comparison with the above-mentioned method as far as the simplicity of sample preparation is concerned, but for successively determining the total mercury content and the methyl mercury content two separate reaction units have to be used, since the two reducing agents mutually influence one another. In addition, it turns out in practice that, due to the use of two reducing agents and of two reaction units, the experimental parameters cannot be controlled in a way which permits the mercury content to be determined with the necessary accuracy and reproducibility.
It is therefore an object of the present invention to provide a method and a device which permit by means of a simple treatment of the sample a reliable determination of the total mercury content and of the mercury content in the form of an organic compound.
According to the present invention, this object is achieved by providing a method of the type mentioned at the beginning in the case of which the control unit controls the supply of the mercury gas to the measuring cell in such a way that, in addition to the measurement of the concentrated mercury, mercury gas of the same sample, which has not been subjected to the concentration step, is introduced in the measuring cell for the purpose of measurement.
The present invention is based on the inventors"" finding that when the mercury is converted into the gas phase, e.g. by reduction with sodium hydridoborate, this has not only the effect that cationic mercury is converted into the gas phase, but also that organomercury, such as methyl mercury, is also released as a gaseous compound. The gaseous mercury developing from the cationic mercury can directly be detected in a measuring cell. The gaseous compound developing from the methyl mercury does, however, not cause any specific atomic absorption or atomic fluorescence during this measurement. It follows that, even if the sample solution is treated such that not only gaseous mercury develops from the cationic or inorganic mercury but that a gaseous compound of the organomercury (methyl mercury) develops as well, a measurement in the measuring cell will still exclusively indicate the content of cationic mercury in the sample.
The gaseous compounds produced can be collected on the concentrator, e.g. a noble-metal net or sand coated with noble metal, and amalgamated. Under the catalytic influence of the noble metal, the organic gaseous mercury compounds will decompose. When the mercury has been released from the concentrator and subsequently introduced in the measuring cell, the sum of inorganic (cationic) and organic mercury will therefore be detected. This permits a determination of the content of organic mercury and of the total mercury content by means of two successive measurement steps; a simple sample preparation will suffice for this purpose.
According to an advantageous embodiment, the mercury contained in the sample solution is converted into the gas phase by reduction. Hence, known devices can be used for reducing the sample in the method according to the present invention, whereby the expenditure for the devices required will be reduced.
According to an advantageous further development of the present invention, the sample containing the mercury is reduced with sodium hydridoborate.
In the course of the experiments carried out by the inventors, it turned out that especially the gaseous mercury compounds obtained by a reduction with sodium hydridoborate permit a reliable determination of the inorganic or cationic as well as of the total mercury content.
According to another advantageous further development, the mercury gas will be introduced into the measuring cell without having been subjected to the concentration step, if the control unit has detected that a predefined threshold value has been exceeded in the preceding measurement of the concentrated mercury gas supplied from the concentrator.
Due to the fact that a threshold value is predetermined and that this threshold value is automatically supervised, a swift measurement is guaranteed, since only the second measurement step for determining the inorganic mercury content will be carried out, if an admissible total mercury content has been exceeded.
According to an advantageous embodiment, the control unit determines an overall result on the basis of the measurement results of the two measurements.
In this way it is, on the one hand, possible to automatically determine the total mercury content as well as the amount of inorganic mercury in a sample, and, on the other hand, the content of organic mercury compounds can be determined automatically with due regard to the two measurement results.
The method can be used in an advantageous manner when the organic mercury compound contains methyl mercury.
The method according to the present invention thus permits a simple preparation of a sample containing mercury and the simultaneous determination of the amount of extremely toxic methyl mercury as well as of the total mercury content.
The present invention also provides a device for detecting mercury in a solution, comprising a gas phase area in which the mercury in the solution can be converted into the gas phase, a concentrator for concentrating the mercury, an atomic spectroscope provided with a measuring cell which is implemented such that it is suitable for detecting mercury, a supply line which serves to introduce the mercury gas into the measuring cell, and a control unit communicating with the measuring cell, said device being characterized in that the supply line has provided therein a controllable valve element, which communicates with the control unit, in such a way that, in dependence upon said control unit, the mercury gas of the same sample solution can be introduced from the gas phase area into the measuring cell selectively with or without the concentrator.
It follows that this device according to the present invention is particularly suitable for executing the method according to the present invention, since the valve element permits a controlled supply of a sample gas selectively from the concentrator or from the gas phase area.
The present invention additionally relates to a device for detecting mercury in a solution, comprising a gas phase area in which the mercury in the solution can be converted into the gas phase, a concentrator for concentrating the mercury, an atomic spectroscope provided with a first measuring cell which is implemented such that it is suitable for detecting mercury gas from the concentrator, a supply line which serves to introduce the mercury gas into the first measuring cell, and a control unit communicating with the first measuring cell, said device being characterized by the following features according to the present invention: that the device comprises a second measuring cell which communicates with the control unit and which, under the control of said control unit, is adapted to have supplied thereto mercury gas directly from the gas phase area excluding the concentrator.
The provision of a second measuring cell, which is adapted to have supplied thereto mercury gas directly from the gas phase area, is particularly suitable for the above-mentioned method according to the present invention; an increased measurement speed can be achieved in view of the fact that reference and calibration measurements, respectively, can be carried out in the second measuring cell e.g. during the measurement taking place in the first measuring cell so that a high measurement accuracy can still be achieved in the case of high measurement speeds.
According to an advantageous embodiment, the gas phase area comprises a reduction area in which the mercury can be converted into the gas phase by reduction.
This permits the use of known reduction devices in the device according to the present invention, whereby an economy-priced overall solution will be obtained.
According to an advantageous embodiment, the control unit is implemented such that it is adapted to form on the basis of the measurement of the mercury content of the gas supplied from the concentrator and on the basis of the measurement of the amount of mercury supplied from the gas phase area a total value resulting from both measurements.
When this total value resulting from both measurement processes is being determined, the different pretreatments of the mercury gas, when said mercury gas is fed from the concentrator and directly from the gas phase area, respectively, are taken into consideration. Furthermore, the control unit can constantly cause the execution of a reference measurement in the measuring cell or measuring cells so as to incorporate a shift of experimental parameters in the evaluation and combination of the individual measurements.
A further embodiment is obtained in combination with the atomic absorption spectrometer device disclosed in DE 44 11 441 in which the measuring cell comprises at least two chambers which are adapted to be connected to one another and through which a light beam can be passed, the light absorption paths of these chambers being in a certain ratio to each other.
On the basis of the subdivision of the measuring cell into at least two connectable, separate chambers it is possible to provide different light absorption paths for the sample substance which is to be examined with regard to its mercury content. The chamber having the longer light absorption path may serve to detect the mercury which is directly supplied from the gas phase area into the measuring cell, so as to achieve a higher sensitivity in comparison with the sample supplied from the concentrator into the measuring cell.
Furthermore, it will also be advantageous when, by means of a division into separate chambers or by a suitable selection of the length and, consequently, of the absorption length, the second measuring cell is adapted to the behaviour of the first measuring cell into which the mercury is fed from the concentrator. This measure permits a substantial increase in the accuracy of the measurement and of the total result.
According to an advantageous embodiment, the chambers in the measuring cells are adapted to be connected by switching valves which are controllable by the control unit. An automatic light absorption length of the spectrometer, which is adapted to the respective measurement conditions, can be adjusted in this way.
Further advantageous embodiments are disclosed in the subclaims.